We have demonstrated that following the rejection of an allograft, processed fragments of the donor major histocompatibility antigens (MHC) class II molecules are presented by recipient antigen presenting cells (APCs) to alloreactive T cells. Recently, this finding has been confirmed by other investigators in both human and rat systems. this self-restricted component of the alloresponse may play an important role in perpetuating the rejection of the transplant by providing help for the production of donor-directed antibodies and by continuous lymphokine release leading to delayed type hypersensitivity (DTH) and cytotoxic T cell activity. More recently, by using a series of overlapping MHC peptides progressing along the sequence of the donor MHC molecule in single residue steps, we mapped all potential MHC antigen determinants to which T cell responses could be generated following an allo- transplant. We observed that the T cell response to beta1 domains of allogeneic Ak, Ad and As mouse MHC class II molecules is directed towards a single immunodominant determinant in each of three donor/recipient combinations (G. Benichou et al. Nature, submitted). Therefore, immune interventions such as tolerance induction to the dominant T cell determinant, which have proven to be effective in the therapy of autoimmune disease, may be developed for the prevention of allograft rejection. Many immunosuppressive drugs have been shown to be efficient in blocking rejection during the early and acute phase of the process. Nevertheless, 1) these drugs are blocking non-specifically the entire immune system harming the patient by reducing its protection against infection and cancer cells, 2) these drugs are often toxic, 3) these drugs are often inefficient in the long run at preventing the perpetuation of the rejection process. Peptide therapy may represent a specific and nontoxic way to prevent or delay the rejection of the graft, by targeting the relevant alloreactive T cells without affecting the rest of the immune system. The goals of this project are 1) to investigate the contribution of donor MHC peptides in in vivo allorecognition and their role in allotransplant rejection, 2) to inhibit the self-restricted T cell response to donor-MHC peptides by tolerance induction to the dominant T cell determinants or by blocking the main T cell clones by anti-TCR antibodies in order to prevent, delay or block the rejection of the graft. Specific aim 1: To map the T cell determinant(s) present on murine (H-2) and human (HLA) molecules. Specific aim 2: 2a) To determine the frequency of CD4+ T helper as well as CD8+ cytotoxic T cells directed against donor-MHC peptides. 2b) To determine the frequency of memory T cells specific for donor-MHC peptides and establish their contribution in perpetuating the rejection of the graft. Specific aim 3: To define the T cell receptor (TCR) V genes used by alloreactive T cells directed towards donor-MHC peptides during graft rejection. Specific aim 4: To induce tolerance to the dominant donor-MHC peptides.